1027. Longest Arithmetic Subsequence

https://leetcode.com/problems/longest-arithmetic-subsequence

Description

Given an array nums of integers, return the length of the longest arithmetic subsequence in nums.

Recall that a subsequence of an array nums is a list nums[i1], nums[i2], ..., nums[ik] with 0 <= i1 < i2 < ... < ik <= nums.length - 1, and that a sequence seq is arithmetic if seq[i+1] - seq[i] are all the same value (for 0 <= i < seq.length - 1).

Example 1:

**Input:** nums = [3,6,9,12]
**Output:** 4
**Explanation:** 
The whole array is an arithmetic sequence with steps of length = 3.

Example 2:

**Input:** nums = [9,4,7,2,10]
**Output:** 3
**Explanation:** 
The longest arithmetic subsequence is [4,7,10].

Example 3:

**Input:** nums = [20,1,15,3,10,5,8]
**Output:** 4
**Explanation:** 
The longest arithmetic subsequence is [20,15,10,5].

Constraints:

• 2 <= nums.length <= 1000

• 0 <= nums[i] <= 500

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